1. Field of the Invention
The present invention is directed to a method and system for reactively converting a liquid fuel into a gasified stream. More particularly, the method and system of the present invention provide a novel means for converting the liquid fuel into a gas by catalytic partial oxidation. In addition, when fuels containing sulfur are used, a method and device according to the present invention can be employed to provide de-sulfurization.
2. Brief Description of Related Art
Gasification of liquid fuels, referred to as “liquid fuel reforming,” typically comprises use of a vaporizer. Vaporization of liquid fuels (e.g., alcohols, hydrocarbons) typically is achieved by indirectly supplying heat into a stream of liquid fuel via heat exchange with a hot wall. One disadvantage of this method is that the rate of vaporization is limited by the rate of heat transfer such that a relatively large surface area is required for fuel vaporization. Another disadvantage of this method, especially for vaporizing long chain hydrocarbons, is that heating the fuel stream to the vaporization temperature tends to cause fuel decomposition and formation of deposits. More specifically, coke formation is problematic. Moreover, preventing deposits from forming within fuel passages in a liquid fuel delivery nozzle during steady state process operation is challenging, due to heat-up of the nozzle from the downstream reaction zone (“hot zone”).
Another known method for gasification of a fuel stream comprises mixing atomized fuel with a hot gas such as superheated steam that supplies the heat required for fuel vaporization and prevents coke formation. However, the large amounts of superheated steam provided by an external steam source required in this method result in a large heat load for steam production.
Spray methods for atomization of liquid fuels known in the art include air-blast or pressure atomizers, ultrasonic and electrospray atomizers. These spray systems are capable of providing a uniform distribution of atomized fuel across the entrance of a catalyst bed in a reforming reactor. Such atomizers may include a co-flow of air that allows mixing of the fuel and oxidizer; however, very fine and uniform droplet size and homogeneous fuel-air distribution, required to avoid coke formation and obtain temperature/mixture uniformity in the reactor, are difficult to achieve in practical spray systems.
Ignition devices, such as spark or glow plugs, are widely used to ignite fuel-oxidizer mixtures at startup. These devices often are subject to failure due to their exposure to high operating temperatures by virtue of their location required for ignition.
Monoliths are commonly used catalyst substrates for the gasification of liquid fuel. Inhomogeneities in a fuel-oxidizer mixture are usually detrimental to monolith substrates leading to localized lean or rich zones, respectively, causing hot spots or carbon precipitation regions. Little opportunity exists for these zones to re-mix, because the channels in the monolith substrate are long and separated from each other; thus monolith substrates are particularly vulnerable. In addition, carbon deposition is favored in monoliths due to build-up of boundary layers that develop on the surface of channels in these substrates.
Combustion of liquid fuels in fuel cell or internal combustion engine systems poses significant problems, especially for fuels with high aromatic content and wide boiling point distribution. This can be attributed to the propensity of heavier aromatic compounds in the fuel to form deposits or coke when vaporized at high temperatures.
Liquid hydrocarbon fuels such as gasoline, kerosene or diesel may be used with high temperature solid oxide fuel cells (“SOFC”) to directly produce electric power. For SOFC fuel cells, the choice of fuel is not limited to pure hydrogen as is the case for low temperature proton exchange membrane (“PEM”) fuel cells. Conversion of the hydrocarbon fuel into a gaseous mixture containing hydrogen (H2) and carbon monoxide (CO) (hereinafter “syngas”) is required before the fuel may be fed to the SOFC. Furthermore, removal of sulfur normally contained in the fuel is needed prior to feeding the gaseous reformate to the SOFC.
U.S. Pat. No. 4,255,121 (hereinafter “Sugimoto”) discloses a reforming process and apparatus to produce a gaseous fuel. The process involves atomizing a mixture of fuel and liquid water; feeding air to the atomized mixture and heating the resulting mixture of misted fuel, air, and water; partially-combusting the heated mixture over a bulk metal catalyst; adding additional air to the partially-combusted mixture and burning in flame. Heating is obtained with a conventional heating element wound around the length of the apparatus including the area surrounding the nozzle where fuel is introduced. Moreover, the bulk metal catalyst inherently has low surface area and thus unacceptable catalytic activity. Sugimoto fails to teach steam production and heat integration; and the cited process is not self-sustaining (autothermal).
U.S. Pat. No. 7,037,485 B1 (hereinafter “Drnevich, et al.”) discloses a multi-component chemical plant for steam reforming methane. In a first reactor, a feed stream of natural gas and optionally olefinic hydrocarbons is reacted over a catalyst capable of promoting either hydrogenation or partial oxidation. Heat from the reaction is collected in a heat exchanger and used to produce steam, which is stored in a steam drum. The stored steam is later fed into a second reactor, specifically a natural gas-tail gas reformer, to produce hydrogen. Drnevich, et al. does not disclose feeding steam so produced back into the first reactor for heat integration or self-sustenance.
These and other known methods and systems for gasification of liquid fuels are described further in U.S. patent application Ser. No. 10/902,512, filed on Jul. 29, 2004, now published as US 2005/0028445 A1 (hereinafter “Roychoudhury, et al.”). Specifically, Roychoudhury, et al. discloses a method and system for gasification of a liquid fuel involving contacting a fuel-oxidant mixture in a short-contact-time, ultra-short-channel-length metal substrate catalytic reactor. Roychoudhury, et al. fails to disclose internal production of steam and use thereof in providing for a self-sustaining process.
U.S. Pat. No. 5,051,241 (Pfefferle) discloses a Microlith® ultra-short-channel-length catalytic reactor having flow channels less than about one millimeter in length and having a ratio of channel length to channel diameter of less than about 2/1.
U.S. Pat. No. 5,069,685 (Bissett, et al.) discloses coal gasification involving a hot fuel gas desulfurization step.
An improved gasification and pre-reforming of liquid fuel would resolve many of the issues noted above with respect to the prior art. It would therefore be desirable to provide a pre-reforming reactor for partially oxidizing and cracking heavy hydrocarbon components of a range of liquid fuels. The pre-reformed fuel, which would be rich in hydrogen and carbon monoxide, subsequently could be further reformed or combusted to power fuel cell systems, internal combustion engines, burners, and other energy-producing devices.
In such a gasifier or pre-reformer (if the reformed fuel is to be further reformed), it would be desirable to provide cold vaporization of the liquid fuel, so as to eliminate the conventional large and costly vaporizer and to avoid formation of coke deposits. The term “cold” as used herein shall mean that the fuel entering the fuel delivery nozzle and until it exits the nozzle remains at a temperature below the coking temperature of the fuel. Preferably, the fuel entering the nozzle and until it exits the nozzle remains at a temperature ranging from about −20° C. to about 50° C. Accordingly, it would also be desirable to achieve rapid start-up with a cold fuel, which shall be taken to mean a start-up time ranging from about 15 seconds to about 1.5 minutes to reach steady state operation. It would also be desirable to provide a method whereby no external pre-heating of either air or fuel is required.
It is still further desirable to provide a catalyst substrate that facilitates mixing of the stream flowing there through, so as to minimize as much as possible rich or lean zones. Such a configuration would result in a comparatively high conversion rate of the reactants selectively to the desired products, would help to minimize high and low temperature regions, and minimize breakthrough of unreacted fuel.
It would also be desirable to provide a catalytic reactor for the gasification of liquid fuels comprising a catalyst that yields partial oxidation products, preferably, CO and H2 in contrast to complete oxidation products, namely, carbon dioxide (CO2) and water (H2O). This results in a higher selectivity to desirable products (CO+H2) for the same amount of added air and produces hydrogen-rich gas directly from the gasifier reactor. It would be further desirable to add steam to the reforming process to control the quantity of hydrogen produced without, however, increasing the energy and cost burdens of producing steam.
It would also be desirable to achieve a steady state operation such that the reforming reaction does not run-away from too much heat production with consequential unacceptable increase in CO2 and H2O yields, or alternatively, does not slow-down from inadequate heat production with consequential coking and unacceptable liquid fuel conversion.
Lastly, it would be beneficial to provide de-sulfurization of the gaseous reformate when required by a particular application of the gasification system taught herein. Desulfurization with zinc oxide, as commonly known in the art, requires control on the temperature of the reformate stream entering the desulfurization unit. Notably, the inlet temperature to the desulfurizer should be less than about 400° C.